Forces and
Freebody Diagram Notes
The Meaning of Force
A force is a push or pull upon an object resulting
from the object's interaction with another
object.
Force is a quantity which is measured using the
standard metric unit known as the Newton.
Types of Forces
Applied Force - Fapp
• An applied force is a force which is applied to
an object by a person or another object.
Types of Forces
Gravity Force (also known as Weight) - Fgrav
• The force of gravity is the force with which the
earth, moon, or other massively large object
attracts another object towards itself.
Fgrav = m * g
where g = 9.8 m/s2 (on Earth) and m = mass (in kg)
Types of Forces
Normal Force - Fnorm
• The normal force is the support force exerted
upon an object which is in contact with
another stable object.
Types of Forces
Friction Force - Ffrict
• The friction force is the force exerted by a
surface as an object moves across it or makes
an effort to move across it.
Types of Forces
Air Resistance Force - Fair
• The air resistance is a special type of frictional
force which acts upon objects as they travel
through the air.
Types of Forces
Tension Force - Ftens
• The tension force is the force which is
transmitted through a string, rope, cable or
wire when it is pulled tight by forces acting
from opposite ends. The tension force is
directed along the length of the wire and pulls
equally on the objects on the opposite ends of
the wire.
Types of Forces
Spring Force - Fspring
• The spring force is the force exerted by a
compressed or stretched spring upon any
object which is attached to it. An object which
compresses or stretches a spring is always
acted upon by a force which restores the
object to its rest or equilibrium position.
Drawing Freebody Diagrams
Free-body diagrams are diagrams used to show
the relative magnitude and direction of all
forces acting upon an object in a given
situation.
Practice with Freebody Diagrams
1. A book is at rest on a table top.
2. A girl is suspended motionless from the ceiling by two ropes.
3. An egg is free-falling from a nest in a tree. Neglect air resistance.
4. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity.
Consider air resistance.
5. A rightward force is applied to a book in order to move it across a desk with a rightward
acceleration. Consider frictional forces. Neglect air resistance.
Practice with Freebody Diagrams
6. A rightward force is applied to a book in order to move it across a
desk at constant velocity. Consider frictional forces. Neglect air
resistance.
7. A college student rests a backpack upon his shoulder. The pack is
suspended motionless by one strap from one shoulder.
8. A skydiver is descending with a constant velocity. Consider air
resistance.
9. A force is applied to the right to drag a sled across loosely-packed
snow with a rightward acceleration.
10. A football is moving upwards towards its peak after having been
booted by the punter.
11. A car is coasting to the right and slowing down.
Determining Net Force
An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction
unless acted upon by an unbalanced force.